Projected capacitive touch sensors typically include a substrate upon which sensing electrodes are disposed. The substrate may be a durable glass having high optical transparency for viewing images displayed by an underlying display device that displays images such as graphical buttons and icons. When a user touches the outer surface of the substrate at a location corresponding to a desired selection displayed on the display device, the location is determined by sensing a change in the capacitance of the sensing electrodes.
In some projected capacitive touch sensors, the sensing electrodes are arranged in rows and columns. The rows and columns comprise pads that are generally arranged in the form of a matrix. Horizontally adjacent pads in a given row of the matrix are connected together to form a single horizontally arranged electrodes may be split so that they do not span the entire sensor. Likewise, vertically adjacent pads in a given column are connected together to form a single vertically arranged electrode, and vertical electrodes like horizontal electrodes optionally may be split.
Typically commercial projected capacitive touch sensor products are constructed from a lamination of a least two layers of glass in which horizontal electrodes and vertical electrodes are on different glass surfaces. For example, horizontal electrodes may be on one surface of a glass layer and the vertical electrodes or the opposite surface of the same glass layer. Alternatively horizontal and vertical electrodes may be fabricated on decent glass layers. In either case, there is manufacturing cost associated with lamination of more than one piece of glass and with the fabrication of electrodes on more than one surface. Alternate designs in which both horizontal and vertical electrodes are fabricated on only one glass surface promise reduced manufacturing cost, particularly if the projected capacitive touch sensor includes only one glass layer with no lamination.
To facilitate both horizontally arranged and vertically arranged electrodes on a single surface, bridging connections may be utilized to connect adjacent pads of a given electrode orientation. For example, bridging connections may couple the vertically adjacent pads that form the vertically arranged electrodes. Known bridging connections have a substantially square geometry. That is, the width and height of the bridge connections are the same.
Associated with each electrode is resistance and capacitance, both of which depend on the size of the touch sensor. As the linear dimensions, of the touch sensor increase, so do the resistances and capacitances associated with the electrodes. The resulting resistor-capacitor time constant (RCtc) representative of electronic settling times of the touch sensor tend to grow quadradically with touch sensor size as both resistance and capacitance grow linearly. For small projected capacitive touch sensors used in smart phones or tablet computers, electronic settling times are less of an issue. However, for touch sensors designed for 15″ diagonal displays and larger displays, long touch sensor electronic settling times become more problematic.
One problem with such large projected capacitive touch sensors is that the resistor-capacitor time constant (RCtc) of the horizontally and vertically arranged electrodes tends to be high and do not match. For example, a typical RCtc for such a large projected capacitive touch sensor may be 9 μs or higher. This is especially problematic when used in conjunction with fixed drive frequency controllers in which the total scan time is determined by the maximum RCtc of the arranged electrodes. The higher the RCtc, the more time that is needed to sense a capacitance value of the electrode. This in turn impacts the rate at which a touch location can be determined, which may negatively impact user experience.
Electronics may readout projected capacitive touch sensitive devices in either self-capacitive mode, mutual-capacitive, or a mixed mode, which is combination of the two. In self-capacitive mode, electronics measure one capacitance per electrode. In mutual capacitance mode, or all-points-addressable (APA) mode, electronics measure capacitance between a row electrode and a column electrode. In either case, the capacitance changes when a finger approaches the electrode. The same projected capacitive touch sensor construction may support self-capacitive mode, mutual-capacitive mode, and mixed mode electronic read out.